Many refineries devote extraordinary amounts of energy and operating expense to convert most of a whole crude oil feed into high octane gasoline. The crude is fractionated to produce a virgin naphtha fraction which is usually reformed, and a gas oil and/or vacuum gas oil fraction which is catalytically cracked to produce naphtha, and light olefins. The naphtha is added to the refiners gasoline blending pool, while the light olefins are converted, usually by HF or sulfuric acid alkylation, into gasoline boiling range material which is then added to the gasoline blending pool.
Catalytic cracking started as a fixed bed process, then evolved to a moving bed process, and finally a fluid bed process, with considerable competition and overlap as each improvement displaced earlier cracking processes. The fluid bed process itself underwent considerable evolution, going from a long folded riser cracking with limited conversion and 10-60 seconds of residence time, to dense bed cracking with increased conversion and residence time, to riser cracking with 1-10 seconds of residence time.
The fluid catalytic cracking (FCC) process is now the preferred process in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline. A finely divided solid cracking catalyst promotes cracking reactions. The catalyst is in a finely divided form, typically with a particles of 20-100 microns, with an average of about 60-75 microns. The catalyst acts like a fluid (hence the designation FCC) and circulates in a closed cycle between a cracking zone and a separate regeneration zone.
In FCC, fresh feed contacts hot catalyst from the regenerator in a reactor. The cracked products are discharged and separated and cracking reactor vapor sent to a main fractionator which produces several product streams. The catalyst is regenerated, and reused.
Depending on the size of the unit, from a few tons to 50 or 60 tons per minute of regenerated catalyst meets feed in the reactor. Each day most units remove a few to about 50 tons of catalyst from the unit, and usually refer to it as E-cat, or equilibrium catalyst, and replace it with fresh or makeup catalyst. As feeds get worse, catalyst removal and makeup rates usually increase, primarily because of metals contamination.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts," Venuto and Habib, Marcel Dekker, N.Y., 1978, incorporated by reference.
Modern refiners are faced with several dilemmas. They must process large amounts of crude oil, frequently containing so much metal that the spent catalyst becomes a disposal problem. Large refineries also generate significant amounts of slop or sludge streams which are very small in terms of crude throughput, but so contaminated with metals and solids that processing of the slop streams is difficult. As an example, many refinery sludges contain so much metal and/or solids that they cannot be cracked in the FCC unit, but these same sludges contain a lot of potentially recoverable hydrocarbon.
In addition, refineries produce other hazardous waste streams such as spent caustic, various spent amine solutions (diethanolamine, DEA and/or methanolamine, MEA). Spent caustic solutions are now hauled away at great expense. It is difficult to even concentrate some of these streams because of corrosiveness and/or fear of plugging equipment with salts.
Activated carbons and various resin catalysts and other absorbents are used in many refineries, and all of these materials become contaminated with metals and/or hydrocarbons during use. They represent another large, and expensive, disposal problem.
I discovered an efficient way to upgrade these waste refinery streams, by making unusual use of another by-product of a refinery, the tons per day of equilibrium catalyst removed from the FCC unit. Catalyst which must be removed from the FCC unit, usually because it has too much metal on it, makes an efficient slop oil upgrader with a large capacity for metals and solids.
In a preferred embodiment, I use not only the adsorbent value of the catalyst, but also make productive use of its thermal energy for upgrading waste streams. Thus, a spent caustic stream can be discharged at relatively low temperature into a vessel containing hot E-Cat. The caustic will be neutralized on the acid sites of the FCC catalyst, which reduces the acidity of the FCC catalyst. Water in the spent caustic is recovered as steam, while hydrocarbons and/or sulfur compounds are generally recovered as hydrocarbon vapor products or H2S. The spent caustic can be added at a relatively low temperature, because the hot FCC catalyst efficient heats even cool caustic streams. The net effect of all this is to make the caustic disappear, and reduce the acidity of the spent FCC catalyst. A waste stream has been eliminated, and the FCC catalyst is no more difficult to dispose of after eliminating the spent caustic Many refinery solid and liquid streams can be eliminated in this way, using spent FCC catalyst as a treating medium.